The primary research objective is to determine the relative contributions of genetics and environment to blood pressure levels among a group of newborn twins. Blood pressure is measured on days 0, 1, 2, 3, 14 and 1, 3, 6, 9, and 12 months. Zygosity is determined through blood group analysis, placental assessment of chorionicity, and clinical assessment of the infants' physical appearance. Placentation is also analyzed for each twin pair with respect to intravascular communications. Additionally, parents and siblings of the twins are evaluated as to blood pressure, height and weight. The ultimate purpose of this research is to shed light on the determinants of blood pressure levels during infancy.